Truck mounted forklifts, otherwise known as piggyback forklifts, have been known for many years. These forklifts can be mounted on the rear of a carrying vehicle or trailer and transported to customers premises on the back of the carrying vehicle or trailer. Once at the customers premises, the forklift can be dismounted from the carrying vehicle and used to transport goods to and from the vehicle on the customer's premises. These forklifts have been found to be useful particularly when used to transport goods to small businesses that do not have access to a forklift, as the deliveries may be made in a fast and efficient manner whilst at the same time reducing the manual workload required by the operators of the small business.
One such piggy-back forklift is that described in the applicants own UK patent application publication number GB2260119. This application describes a piggyback forklift truck having a multi-lift mast comprising an outer mast section for mounting on the truck, a middle mast section slidably mounted on the outer mast section and an inner mast section slidably mounted on the middle mast section. A carriage carrying a pair of forks is further provided and is in turn slidably mounted on the inner mast section. A first chain mechanism connects the outer mast section to the inner mast section and a second chain mechanism connects the middle mast section to the fork carriage. The first and second chain mechanisms are such that as the middle mast section is raised relative the outer mast section by a pair of hydraulic rams, the inner mast section is caused to rise relative the middle mast section and the fork carriage is caused to rise relative the inner mast section. In this way, a truck mounted forklift having a single pair of rams operable on the outer and the middle mast sections may be used simultaneously to raise both the inner mast section and the forks via the chain mechanisms. Therefore only a single pair of rams is required to operate the mast thereby reducing the overall weight of the forklift. This type of forklift has proved to be very successful in operation.
There is however a problem with the known type of forklift. These forklifts are often required to operate in areas with relatively low overhead clearance. In order to raise the forks of the known type of forklift the mast sections must also be raised at the same time by the rams. This increases the overall height of the forklift which may be unacceptable particularly in areas with restricted overhead clearance. The functionality and usefulness of the forklift is therefore compromised.
One solution to this problem has been the introduction of the so called “freelift” multi-lift masts. These masts are similar to the regular multi-lift masts but differ in the fact that the forks are carried by a freelift system which in turn is mounted on the inner mast section. The freelift system replaces the chains connecting the forks to the middle mast section of the known forklifts and comprises a fluid actuated ram and pulley purchase system connected to the mast section and the forks. The freelift system can raise the forks upwardly by a limited amount relative the inner mast section without increasing the height of the mast simply by operating the fluid actuated ram. In order to lower the forks relative the inner mast section, the fluid actuated ram is released and the forks descend under gravity. In this way, adjustment of the height of the forks is possible even in areas with a low overhead clearance. However, this type of freelift multi-lift mast has been found to be unsuitable for truck mounted forklifts. In order to mount a piggyback forklift onto a truck or trailer, they require what is known as “negative lift” whereby the forks of the forklift engage the truck and the forks are driven downwards to lift the forklift truck clear of the ground up onto the carrying vehicle. Due to the presence of the freelift multi-lift mast, the only force limiting the upwards movement of the forks is the weight of the forks themselves. It is not possible therefore to generate enough negative lift to raise the forklift up onto the vehicle.
One proposed solution to this problem is to provide a locking pin to lock the forks and the inner mast section in a fixed relationship with respect to each other before attempting to mount the forklift onto the vehicle. This has the disadvantage that a considerable amount of time must be spent in aligning the mast section and the forks together before the locking pin may be inserted. Another disadvantage is that the operator of the forklift must alight from the vehicle In order to Insert the locking pin which is inconvenient and further increases the amount of time spent in loading and unloading the forklift from the carrying vehicle. This represents a significant amount of time spent in the mounting of the forklift on the carrying vehicle. Another disadvantage of having to use the locking pins is that these locking pins are prone to loss and damage and should the locking pin be lost or damaged when the forklift is being used offsite, a replacement locking pin has to be provided which can result in the loss of a significant amount of time for the operator of the forklift.
A further situation where negative lift is required is when the piggyback forklift is required to navigate a step that is too large for the wheels of the piggyback forklift to mount alone without assistance. Normally, In order to navigate such a step, the operator of the piggyback forklift will extend the mast carrying carriage to its forwardmost position on the u-shaped chassis so that the forks protrude the maximum distance forward of the forklift. When the mast and forks are in position, the operator of the vehicle drives the vehicle up to the step and lowers the forks down on to the step. The forks are then lowered further until the forklift truck begins to rise relative the step. Once the forklift has risen sufficiently so that the wheels are substantially level with the step, the mast carrying carriage is retracted backwards on the unshaped chassis which has the effect of pulling the entire forklift truck and in particular the front wheels of the forklift truck forwards up onto the step. This can only be achieved by the forklift truck having a mast assembly that is able to provide negative lift. Therefore, when using the known types of piggyback forklift with freelift multi-lift masts the operator of the forklift will have to align the forks with the mast section and insert the locking pin to secure the forks in position so that negative lift may be provided to the forks. The operator of the vehicle may have to navigate a step numerous times during the course of a single delivery. If the operator of the vehicle is forced to align the forks with the inner mast section before dismounting from their vehicle and inserting locking pins each time the step must be navigated, the time required to carry out a delivery will be increased greatly. This is highly undesirable.
Another situation in which negative lift may be required is when the forklift is used to pick up goods that may have been left to rest on soft ground. In certain circumstances these goods may subside into the soft ground thereby making their retrieval more difficult. By having negative lift, the operator of the forklift may use the negative lift to drive the forks of the forklift downwards Into the ground and assist In picking up the pallet or other goods that have subsided. Again, this is only possible with negative lift. If the operator of the vehicle had to move a number of pallets or other goods that had begun to subside then the time taken to effect delivery would be increased significantly as the forks would have to be aligned and the locking pins inserted for each pallet that had to be lifted by the piggyback forklift. It is imperative that the piggyback forklift is able to provide a sufficient degree of negative lift In a quick and efficient manner.